diff --git a/docs/conf.py b/docs/conf.py
index 86a158df..ae65fbbc 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -52,6 +52,7 @@ extensions = [
     "sphinx.ext.doctest",
     "sphinx.ext.intersphinx",
     "sphinx.ext.todo",
+    "notfound.extension",
 ]
 
 
diff --git a/setup.py b/setup.py
index f46a0b8c..9941014b 100644
--- a/setup.py
+++ b/setup.py
@@ -44,7 +44,7 @@ CLASSIFIERS = [
 ]
 INSTALL_REQUIRES = []
 EXTRAS_REQUIRE = {
-    "docs": ["furo", "sphinx", "zope.interface"],
+    "docs": ["furo", "sphinx", "zope.interface", "sphinx-notfound-page"],
     "tests_no_zope": [
         # 5.0 introduced toml; parallel was broken until 5.0.2
         "coverage[toml]>=5.0.2",